This invention relates to milling machines that shape workpieces by the removal of material with rotating chip making and grinding tools. While these machines may be manually or computer controlled, modern computer numerical controlled (CNC) milling machines are increasingly performing the greatest proportion of milling tasks. Typical CNC milling machines have a machine spindle head with a rotating spindle shaft that handles a plurality of machining tools, including drills, endmills, and many styles of chip removing cutters. When these CNC milling machines have an included mechanism for the exchanging of these chip cutting tools, they are generally referred to as machining centers. These milling machines and machining centers are designed to produce a finished workpiece from the raw starting material as quickly and precisely as possible. Great effort has been expended in making machine movements as fast as possible, and in designing milling tools that efficiently remove large quantities of waste material by their cutting action. During the actual machining operations the computer controlled movements of the milling tools through the workpiece are optimized for removing waste material from the workpiece in the form of chips at the maximum rate. When an exchange of tools is required, the interruption of the machining operation for the tool exchange function is typically so short that little time is added to that of actual machining.
The spindles on the most common CNC milling machines have either a vertical or horizontal orientation that sets the manner in which the milling cutters address the workpiece. It is obvious that workpieces may require milling from more than one side, and such workpieces require extra operations which may include repositioning of the workpiece on the machine's workholding table so that the cutters can address the other sides of the workpiece. This repositioning of the workpiece causes a loss of time and accuracy in the operations. Some milling machines have more than one tool driving spindle, with secondary spindles being able to work on the same or other sides of the workpiece. Milling machines that have these secondary spindles are, however, of special and expensive construction, and as such are less common in the industry. The most complex and expensive milling machines respond to these problems by the inclusion of mechanisms which tilt the spindle or the workpiece about one more axes, thus allowing the cutters to address the workpiece form more than one side. These complex machines, often called universal or 5-axis milling machines, while versatile in achieving many angles of milling, lose rigidity in the tilting mechanisms, can accommodate relatively smaller workpieces, and must be constructed with great and costly care to achieve accuracy.